The present invention is directed to apparatus and a method for detecting an unexpected connection in an alarm system such as an alarm communications system having a controller communicating with a plurality of transponders. More particularly, the invention enables the detection and reporting not only of an abnormality which occurs between a specific transponder and a transducer it is monitoring, but also at which transponder the malfunction or abnormality has occurred.
Alarm systems which provide two-way communications between a controller and a plurality of transponders, or modules, are now well-known and widely used for providing life, safety and security protection. Typically, the transponders have one or more transducers connected or coupled to them. For operating such systems, regulatory agencies require that the system have a high level of reliability in detecting and reporting alarm conditions. This is true even if, for example, there is an unexpected connection to earth ground between a transponder and a connected transducer. The transducer can be a temperature sensor, door position sensor, motion detector, or other sensor. It is desirable that the transponder accurately detect and report back necessary system data, even in the event of an unexpected connection to earth ground on a local loop between the transducer and transponder. It is further desirable that the system also report the fact of an occurrence and the location of the ground fault or other anomaly. While most systems can tolerate one ground fault for a limited time, location and repair of the fault is highly desirable before a second fault occurs which may cause the entire system to malfunction.
In order to tolerate this undesirable connection, system designers often decrease the impedances of a transducer's sensing circuits. This normally enhances the stability of the sensing circuit. However, to allow the sensing circuit to identify an undesirable connection, one would want to design the circuit with higher impedances, thereby allowing the sensing circuit to be responsive to outside influences. These two requirements are conflicting. If the circuit is able to "ignore" the fault for reliable alarm reporting, then it is consequently difficult to recognize the presence of the fault, much less determine and report its location.